1. Field of the Invention
The present invention relates to novel thermally stable bisimido polymers, and to processes for the preparation of such polymers.
2. Description of the Prior Art
Polymers produced by reacting an N,N'-bisimide of an unsaturated dicarboxylic acid such as, for example, a N,N'-bismaleimide, with a diprimary diamine are known to this art. Compare published French Application No. 1,555,564. The amounts of N,N'-bisimide and of diamine are selected such that the ratio: ##EQU1## is at least equal to 1; furthermore, it is generally preferred that it be less than 50. Thermally stable resins are thus obtained, which withstand very well severe thermal stresses.
In the aforementioned published French application, it is indicated generally that the preparation of these resins may be carried out in bulk, by heating the reactants which have been intimately mixed beforehand, or else in an inert polar diluent such as dimethylformamide, N-methylpyrrolidone or dimethylacetamide, it being possible for this latter process to be employed, for example, when the application of the polymer requires a solution thereof to be used.
Finally, it is also noted that, for many uses, it is advantageous to conduct the reaction in two stages; in a first stage, a prepolymer is prepared by heating an intimate mixture of the two reactants to a temperature on the order of 100.degree. to 250.degree. C. The prepolymer obtained may be used in the form of a solution, suspension or powder, or else it can also be shaped merely by being cast hot. In a second stage, curing of the prepolymer is effected by heating same to temperatures on the order of 300.degree. C., optionally under pressure.
These polymers may be converted into films or into polycellular materials. They are of very particular interest for the manufacture either of articles which are molded optionally in combination with fibers or pulverulent fillers, or of laminates based on inorganic fibers (single fibers, fabric or nonwoven fibers) such as, for example, carbon, boron or glass fibers. However, the preparation and the use of these polymers require precautions to be taken so far as health and hygiene are concerned, when the diprimary diamine employed is of aromatic nature, because of the toxicity of certain of these compounds.
Thus, to overcome this disadvantage, need exists for novel polymers containing imide groups which do not require diamines for the preparation thereof.
Polymers containing imide groups which may be prepared without the use of diamines have already been described in European Patent Application EP-A-0,167,304. Such polyamides are produced, for example, from:
(i) a combination of a number of N,N'-bismaleimides; and
(ii) a (meth)acrylic ester consisting of the reaction product of (meth)acrylic acid with an epoxy resin, such as a glycidyl ether of poly(hydroxyphenyl)alkane or of a phenol/formaldehyde polycondensate of the novolak type.
It has now been determined that the disclosure of the above-mentioned patent application does not make it possible, beginning with a reactant generating imide groups consisting of a single N,N'-bismaleimide by itself, to produce homogeneous prepolymers, and polymers in the case of which the bismaleimide and the (meth)acrylic ester are subjected to a joint copolymerization reaction. It has also been ascertained that the prepolymers which can be prepared are ill-suited for the production of preimpregnated intermediate articles (used for the manufacture of laminates) when the solvent-impregnation technique is employed, because of the very low solubility of these prepolymers in the solvents which are generally employed.